The present invention regards a microelectromechanical structure insensitive to mechanical stresses.
As known, surface and epitaxial micromachining techniques allow production of microstructures within a layer that is deposited (for example a polycrystalline silicon film) or grown (for example an epitaxial layer) on sacrificial regions that are removed at the end of the manufacturing process by wet etching.
In general, the layers subject to manufacturing (deposited or grown layers) are formed at high temperatures, completely different from the operative temperatures. In addition, the various regions forming the end devices have different thermal expansion coefficients. Consequently, at the microstructure operative temperatures, residual mechanical stresses are present; in addition, in particular when the various regions are doped not uniformly, the stresses are not uniform (stress gradients); these stresses thus cause undesirable mechanical deformations of the microstructures, as described schematically hereinafter with reference to FIGS. 1-6.
In detail, FIG. 1 shows in cross-section a structure 1 comprising a polycrystalline silicon bridge element 2, formed on a monocrystalline silicon substrate 3; a sacrificial oxice layer 4 extends between the bridge element 2 and the substrate 3, except for two areas, where anchorage portions 5 of the bridge element 2 extend through the sacrificial oxide layer 4, and are supported directly on the substrate 3.
FIG. 2 shows the same structure 1 as in FIG. 1, in plan view.
FIGS. 3 and 4 show the structure 1, after removal of the sacrificial oxide layer 4, when the dimensions of the structure have been reduced (shown exaggerated in the figures, for better understanding), owing to the presence of residual stress; in particular, in FIG. 3, owing to the different thermal coefficient, the dimensions of the bridge element 2 are reduced (shortened) more than those of the substrate 3; here the bridge element 2 is subjected to tensile stress, and assumes a more favorable energetic configuration. In FIG. 4 on the other hand, the bridge element 2 undergoes a lesser reduction of dimensions than the substrate 3; consequently, in this condition, the bridge element 2 tends to be lengthened in comparison with the substrate 3, but, owing to the fixed anchorage portions 5, it undergoes stress of a compressive type, causing buckling deformation.
In the case of tensile stress, the mechanical resonance frequency of bridge element 2 is shifted upwards with respect to the intrinsic value (in the absence of stress); on the other hand, in the case of compressive stress, the mechanical resonance frequency of the bridge element 2 is shifted downwards.
The average residual stress thus has the effect of modifying the resilient constant of the micromechanical structures; this modification is not reproducible, and can cause mechanical collapse of the structure (in particular in the case in FIG. 4).
In FIG. 5, the projecting element 11 is formed on a monocrystalline silicon substrate 12; a sacrificial oxide layer 13 extends between the projecting element 11 and the substrate 12, except for an area, where an anchorage portion 14 of the projecting element 11 extends through the sacrificial oxide layer 13, and is supported directly on the substrate 12.
FIG. 6 shows the structure 10 of FIG. 5, after removal of the sacrificial oxide layer 13. As can be seen, the release of the residual stress gradient causes the projecting element 11 to flex. In particular, indicating with "sgr"R(z) the function linking the residual stress with the coordinate z in the projecting element 11, {overscore ("sgr")}R the average residual stress xcex93 the strain gradient, and E Young""s modulus, the following is obtained:
"sgr"R(Z)={overscore ("sgr")}R+xcex93Ez 
In addition, indicating with L the length of the projecting element 11, flexure at its free end is independent from the thickness, and is:
H=xcex93L2/2 
Consequently, a positive strain gradient xcex93 causes the projecting element 11 to bend away from the substrate 12 (upwards), whereas a negative gradient causes it to bend downwards.
In case of suspended masses, the behavior is exactly the opposite, i.e., positive stress gradients cause downward flexing, and negative stress gradients give rise to upward flexing.
In addition, the material of the package has a different coefficient of thermal expansion as compared to the material of the micromechanical structure (mono- or polycrystalline silicon). Consequently, the suspended masses may be subject to small displacements with respect to the fixed region of the micromechanical structure.
The presence of residual stress inherent to the structural material and stresses induced by the packaging material jeopardizes the performance of integrated micro-electromechanical devices.
For example, in the case of integrated micromechanical structures having a suspended mass, or seismic mass, provided with a plurality of anchorage points, the stresses inherently present in the materials or induced by packaging, by acting in different and non-uniform way on the various anchorage points, causes tension in some parts and compression in other parts, such as to modify the mutual positions of these parts and to generate non-symmetrical geometries of the structures.
For example, consider the case of an angular accelerometer with a suspended mass having an annular shape set outside the center of gravity of the suspended mass, so as to have a high moment of inertia and hence a high sensitivity. Such an accelerometer is illustrated schematically in FIG. 7 and in detail in FIG. 8, showing only one part thereof.
FIG. 7 shows a semiconductor material chip 20 housing an angular accelerometer 21 comprising a rotor 22 and a stator 23. The chip 20 may moreover house circuit components (not shown) for biasing, controlling the processing signals. The angular accelerometer 21 has a barycentric axis G (defined as an axis passing through the center of gravityxe2x80x94not shown) coinciding with the axis of symmetry of the accelerometer. The rotor 22 (which is able to perform micrometric rotations about the barycentric axis G, in such a way that every movement of the rotor is defined by instantaneous vectors perpendicular to the barycentric axis G) comprises a suspended mass 25 having an annular shape concentric to the barycentric axis G and bearing a plurality of mobile electrodes 26 extending radially inwards from the suspended mass 25. Each mobile electrode 26 is associated with two fixed electrodes 27, 28 extending radially, each of which faces a different side of the respective mobile electrodes 26. The fixed electrodes 27, 28, forming together the stator 23, in practice define, together with the respective mobile electrodes 26, a plurality of capacitive circuits; namely, all the fixed electrodes 27, arranged on first sides (for example, on the left in the clockwise direction) of the respective mobile electrode 26, form first capacitors with the respective mobile electrodes, whilst all the fixed electrodes 28, arranged on second sides (for example, on the right in the clockwise direction) of the respective mobile electrodes 26, form second capacitors with the respective mobile electrodes. The first capacitors are connected in parallel with each other and the second capacitors are also connected in parallel with each other. The first capacitor and the second capacitor associated with the same mobile electrode 26 are, instead, connected in series.
In a per se known manner, any movement of the suspended mass 25 brings about an increase in the capacitance of one of the two capacitors associated to each mobile electrode 26 and a reduction in the capacitance of the other capacitor. Consequently, by appropriately biasing the mobile electrodes 26 and the fixed electrodes 27, 28 and by connecting them to a circuit that measures the capacitance, it is possible to detect any movement of the suspended mass 25 with respect to the stator 23.
In the accelerometer 21 of FIG. 7, the suspended mass 25 is supported and biased by a suspension structure comprising springs 30 and rotor anchorage regions 34. The springs 30 are arranged at 90xc2x0 with respect to one another and extend radially between sets of mobile electrodes 26 and fixed electrodes 27, 28. In particular, as is better illustrated in the detail of FIG. 8, each spring 30 comprises a pair of lateral arms 31 extending radially inwards from the suspended mass 25, at a distance from one another, and connected to one another at their radially inner ends by a cross portion 32. A central arm 33 thus extends radially from the center of the cross portion 32 between the side arms 31 for approximately one half of the length of the sidearms 31 and terminates at the rotor anchorage region 34, which is integral with the chip 20. The springs 30 are suspended and hence are deformable as a result of the rotation of the suspended mass 25.
FIG. 8 moreover shows stator anchorage regions 35a, 35b integral with the fixed electrodes 27 and 28, respectively. The stator anchorage regions 35a, 35b have a width greater than the fixed electrodes 27, 28, extend in depth as far as the substrate (in a not shown manner), and protrude on the sides of the fixed electrodes 27, 28 not facing the respective mobile electrodes 26. In FIG. 8, a trench 38 separates the mobile mass 25 from the rest of the chip 20.
With the angular accelerometer of FIG. 7, the problem arises that residual stresses or stresses linked to the materials and acting on the rotor 22 cause tensions and compressions of the springs 30, deforming the rotor 22 in a non-foreseeable way.
The angular accelerometer 21 thus presents reduced performance in terms of sensitivity and precision. In addition, the variations in performance are non-uniform on components belonging to different batches and, at times, on components belonging to a same batch.
An embodiment of the invention is directed to a microelectromechanical structure that includes a first plurality of stator elements and a rotor having a baycentric axis. The rotor includes a central anchor portion through which the barycentric axis extends; a first seismic mass separated from the central anchor portion; a first plurality of mobile rotor elements interleaved with the plurality of stator elements, the rotor elements extending from and being supported by the first seismic mass; and a first plurality of flexible support arms extending between the first seismic mass and the central anchor portion, the support arms flexibly supporting the first seismic mass such that the seismic mass and rotor elements are movably coupled to the central anchor portion. The microelectromechanical structure can implement various devices, including an angular accelerometer, a linear accelerometer, and a gyroscope.